Consolidate UI page widths - Part 01: Repository

Problem to solve

Via this design pattern issue we explored how to consolidate our page widths (990px, 1280px, full-width). The overall goal is to simplify and move most pages to a width of 990px.

In addition, we are tackling a second iteration https://gitlab.com/gitlab-org/design.gitlab.com/issues/103 to identify if we can get right of the larger width 1280px completely by moving these to either the smaller width or full width.

Proposal

This issue serves as the corresponding implementation part. I expect that related changes will be split into several MRs and potentially be implemented across a few milestones.

Scope: All sub-pages of "Repository"

What does success look like, and how can we measure that?

We reduced the available range of page widths to simplify FE environment, reduce complexity and improve readability in the UI.